The most likely thing is that it is the salad itself. Most lettuces are quite bitter plants, naturally. Modern supermarket varieties are bred for removing that bitterness, but it is not infallible, and you will still get the occasional bitter leaf. Heirloom varieties and less widespread salad plant species tend to be outright bitter. Also, endives are a widespread species which tends to be bitter even in the supermarket.
It is also normal to get a bit of a habituation for the bitter taste after the first bite. Bitterness can be an enjoyable acquired taste, even though it still catches one by surprise at the first bite.